1. Field of the Invention
The present invention relates to threshold amplifiers and more particularly to threshold amplifiers suited to integrated circuit fabrication by a process employing complementary metal oxide semiconductor (CMOS) field effect transistor technology.
2. Description of the Prior Art
Integrated circuits have traditionally fallen into separate classes: those which operate in a digital fashion and those which operate in an analog fashion. The complementary metal oxide semiconductor field effect transistor process has traditionally been used for digital applications in which both the input signals and the output signals are digital. The present invention deals with the problem of creating an interface between analog data which may be obtained by sensors not a part of the integrated circuit and digital logic which is a part of the integrated circuit.
In this effort, it is essential that a means be provided for determining how to deal with analog data. Such means include the threshold amplifier which, in the ideal case produced either a "1" or a "0" in the output, while the input continuously varies through a threshold value.
The present threshold amplifier utilizes, as a circuit portion, a "mirror" that has as its counter part in analog circuitry, the bipolar current mirror. In the conventional bipolar current mirror, a diode connected transistor is provided in an energized current path, and it is connected across the input junction of an active bipolar transistor, whose operating point is to be stabilized. Base current flow in the bipolar transistor and temperature effects tend to reduce the stability of the operating point, and other circuit measures usually are adopted to compensate for these limitations of the circuit. The present invention employs the principle of the mirror in a field effect transistor configuration.